US 2014/0090356 A1 has disclosed a heat recovery steam generator having a plurality of heat exchangers, including a so called superheater, an evaporator and an economizer. The heat exchangers are arranged in a housing (i.e. a duct section) in the flow direction of an exhaust gas from a gas turbine. Steam is generated utilizing the heat of the hot turbine exhaust gas. This known heat recovery steam generator further comprises auxiliary combustors, which are each disposed upstream of one of the heat exchangers, for further heating the exhaust gas by means of burners. An air supply device for additionally supplying air to the burners of at least one of the auxiliary combustors, in particular the most downstream one, from the outside of the duct is also provided. In an embodiment shown a second-stage auxiliary combustor is positioned as seen in the direction of flow of the turbine exhaust gas downstream of a first-stage auxiliary combustor. A high-temperature and a low-temperature superheater for generating steam are positioned between the auxiliary combustors. Both combustors are fed with fuel. As the oxygen content in the exhaust gas downstream of the first-stage combustor may be insufficient for a stable combustion at the second-stage auxiliary combustor additional air is fed to the second-stage auxiliary combustor by means of a fan and suitable ducting. The amount of air to each burner of the second-stage combustor is individually controlled by air control valves. According to this patent document in this way it is possible to maintain an optimal combustion state at the second-stage combustor.
For a stable combustion both temperature and oxygen content are significant parameters. The lower the temperature, the higher the oxygen concentration should be. And the lower the oxygen content, the higher the temperature. In the heat recovery steam generator the ratio of oxygen to temperature is increased by the introduction of fresh air having an oxygen content of about 17-21% compared to an oxygen content of about 8-16% in the turbine exhaust gas.
A drawback of this known heat recovery steam generator is that relatively cold air is introduced, which needs to be heated to the appropriate combustion temperature by the hot exhaust gas, which reduces the efficiency. Thus the efficiency of the heat recovery system leaves something to be desired. Furthermore the expenses of the air fan and associated air supply system and equipment and its operation add to the total costs and require additional electrical power. Furthermore failure of this air fan and associated equipment might result in interruption of the operation of the heat recovery steam generator.